The present invention relates to methods for the treatment of fluorine-containing waste waters.
As the conventional method for fixing and removing fluorine values from fluorine-containing waste waters, there is known a method for the treatment of fluorine-containing waste waters comprising adding calcium compound (such as slaked lime, limestone, gypsum or the like) to such waste water to cause reaction represented by the following formula (1): EQU Ca.sup.2.sup.+ + 2F.sup.- .fwdarw. CaF.sub.2 ( 1)
thus precipitating fluorine values in the waste water in the form of a hardly soluble salt, namely calcium fluoride, and separating this salt from the waste water.
According to this known method, however, it is impossible to reduce the fluorine concentration below about 8 ppm, and depending on the fluorine concentration in waste water to be treated, the amount added of Ca.sup.2.sup.+ should be drastically increased. When the amount added of Ca.sup.2.sup.+ is increased, the quantity of sludge is inevitably increased and unreacted Ca.sup.2.sup.+ is often left in the treated waste water, causing undesired phenomena.
Recently, it is desired to reduce the fluorine concentration below 4 ppm or below 1 ppm in some special cases, and this desire cannot be satisfied at all by the above-mentioned conventional method comprising addition of a calcium compound.
The residual fluorine concentration in waste waters treated according to the conventional method using a calcium compound is shown in FIG. 1. As is seen from this FIG. 1, according to this conventional method, the residual fluorine concentration is reduced to about 8 ppm at lowest regardless of the initial fluorine concentration, and when the initial fluorine concentration in waste water to be treated is lower than 40 ppm, the amount added of Ca.sup.2.sup.+ should rather be increased so as to lower the fluorine concentration to 8 ppm. This tendency is more conspicuous in the case of actual waste water practically discharged from an industrial plant, which contains various pollutants and substances in addition to fluorine, than in the case of model waste water containing fluorine alone, as shown in FIG. 2. Moreover, it is apparent that when the initial fluorine concentration is as low as about 10 ppm, even if Ca.sup.2.sup.+ is added in great excess, calcium fluoride is not precipitated and fluorine in waste water is not removed at all.
As another method for fixing and removing fluorine values in waste waters, there has been proposed a method comprising addition of phosphoric acid or phosphate, the removal characteristics of which are shown in FIG. 3. Indeed, this method is effective for reducing the fluorine concentration, but phosphoric acid ions should be added in an amount of at least 3 moles per mole of the fluorine values in waste water to be treated. The reason is that the fluorine-fixing reaction in this method is a reaction forming fluoride apatite [ 3Ca.sub.3 --(PO.sub.4).sub.2.sup.. CaF.sub.2 ], which is represented by the following reaction formula (2): EQU 10Ca.sup.2.sup.+ + 6PO.sub.4.sup.3.sup.- + 2F.sup.- .fwdarw. 3Ca.sub.3 (PO.sub.4).sub.2.sup.. CaF.sub.2 .dwnarw. (2)
as is seen from the above reaction formula and experimental results shown in FIG. 3, in this known method using phosphoric acid or phosphate (addition of phosphoric acid ions), when the fluorine concentration is high in waste water to be treated, the amount added of the precipitating agent should be drastically increased.
In view of the foregoing state of the art, we made research works and as effective means for fixing and removing fluorine values in waste waters, we previously developed a method including addition of aluminum ions (addition of soluble aluminum salt or addition of aluminum ions by aluminum electrolysis). According to this method, aluminum ions are added to fluorine-containing waste water at pH 5 to 8 (the optimum pH range) to thereby fix fluorine in the waste water as a hardly soluble hydroxy-fluoride complex, and this complex is removed from the waste water. This method is effective as means for fixing and removing fluorine values. However, as is seen from FIG. 4, it was found that in order to reduce the residual fluorine concentration below 1 ppm, a considerably large amount of the aluminum ion should be added.